Process control plants or systems typically include numerous valves, pumps, dampers, boilers, as well as many other types of well-known process control devices or operators. In modern process control systems most, if not all, of the process control devices or operators are instrumented with electronic monitoring devices (e.g., temperature sensors, pressure sensors, position sensors, etc.) and electronic control devices (e.g., programmable controllers, analog control circuits, etc.) to coordinate the activities of the process control devices or operators to carry out one or more process control routines.
For purposes of safety, cost efficiency and reliability, many process control devices are pneumatically-actuated using well-known diaphragm-type or piston-type pneumatic actuators. Typically, pneumatic actuators are coupled to process control devices either directly or via one or more mechanical linkages. Additionally, the pneumatic actuators are typically coupled to the overall process control system via an electro-pneumatic controller. Electro-pneumatic controllers are usually configured to receive one or more control signals (e.g., 4-20 milliamps (mA), 0-10 volts direct current (VDC), digital commands, etc.) and to convert these control signals into a pressure provided to the pneumatic actuator to cause a desired operation of the process control device. For example, if a process control routine requires a pneumatically-actuated, normally closed stroke-type valve to pass a greater volume of a process fluid, the magnitude of the control signal applied to an electro-pneumatic controller associated with the valve may be increased (e.g., from 10 mA to 15 mA in the case where the electro-pneumatic controller is configured to receive a 4-20 mA control signal). In turn, the output pressure provided by the electro-pneumatic controller to the pneumatic actuator coupled to the valve at least partially increases to stroke the valve toward a full open condition.
In addition to a control signal for indicating a desired set-point of the pneumatically-actuated device (as described in the previous example), the electro-pneumatic controller may be configured to receive a feedback signal from the pneumatically-actuated device. This feedback signal is typically related to an operational response of the pneumatically-actuated device. For example, in the case of a pneumatically-actuated valve, the feedback signal may correspond to the position of the valve as measured by a position sensor. In another example, the position of the pneumatic actuator coupled to the valve may be measured to derive the feedback signal. The feedback signal is typically compared to the set-point, or reference signal, to drive a feedback control loop in the electro-pneumatic controller to determine a pressure to provide to the pneumatic actuator to achieve a desired operation. Feedback control is usually preferred over set-point control alone (also known as open-loop control) because the feedback signal allows the electro-pneumatic controller to automatically counteract or compensate for variations in the controlled process.
The electro-pneumatic controllers used with many modern pneumatically-actuated process control devices are often implemented using relatively complex digital control circuits. For instance, these digital control circuits may be implemented using a microcontroller, or any other type of processor, that executes machine readable instructions, code, firmware, software, etc. to control the operation of the process control device with which it is associated.
To decrease the response time of the process control device, one or more secondary pneumatic power stages may be coupled between the electro-pneumatic controller and the pneumatic actuator. For instance, a secondary pneumatic power stage may include a volume booster and/or a quick exhaust valve. A volume booster increases the amount of or rate at which air is supplied to or exhausted from the pneumatic actuator, which enables the actuator to actuate (e.g., stroke) more quickly the process control device to which it is coupled. Thus, a volume booster may increase the speed at which the actuator is able to stroke a valve to enable the valve to respond more quickly to process fluctuations.
A quick exhaust valve may be coupled between the electro-pneumatic controller and the pneumatic actuator to increase the rate at which air is released or exhausted from a pressurized actuator. Typically, a quick exhaust valve vents air to atmosphere. By increasing the rate at which air is released, the quick exhaust valve enables the actuator to quickly reduce the force applied to the process control device. Thus, a quick exhaust valve may be used to increase the speed at which the actuator can stroke the valve toward a closed or open position.
While secondary pneumatic power stages prove beneficial in decreasing the response time of a pneumatically-actuated device, they may also introduce undesirable transient characteristics in the response of the device. For example, a volume booster may cause a valve to overshoot, in the direction of valve travel, past a desired, steady-state control position. To compensate for such overshoot, the volume booster may then cause the valve to undershoot past the steady-state control position in the opposite direction. In another example, a quick exhaust valve may cause undesirable transient behavior due to its high-capacity, on-off operational response. Moreover, the trip-point for the quick exhaust valve may be highly sensitive and difficult to control, even in the presence of bypasses inserted around the quick exhaust valve. Undesirable transients/control conditions, such as those described above, are typically caused by the delay in the response of the pneumatically-actuated device to variations in the control signal applied the device input, a delay which may be exacerbated by the nonlinear operational characteristics of many secondary pneumatic power stages.